Atomic ordering and magnetic properties in Ni₂Mn(Ga_xAl_1-x) Heusler alloys

The phase stability and magnetic properties of Ni₂Mn(Ga_xAl_1-x) alloys were studied by differential scanning calorimetric and magnetometric measurements, and X-ray diffraction and transmission electronic microscopic observations. The order-disorder transition temperature from the B2 to the L2₁ phase, T_t^{B 2 / L 21}, linearly increases with increasing x and the magnetic properties depend on the annealing conditions. Especially in the Ni₂Mn(Ga_0.5Al_0.5) alloy, the condition of the atomic order can be varied from a B2 to an L2₁ structure by the annealing temperature. The magnetic property of the fully ordered L2₁ phase is ferromagnetic with a Curie temperature T_C of about 380 K, while that in the B2 phase is antiferromagnetic with the Néel temperature T_N of about 300 K.